160 DINOSAURS OF NOETH AMERICA. 



distance is accurately marked on the centrum by a narrow articular 

 border, just back of the flat, anterior face. This peculiar articulation 

 leaves more than three-fourths of the cup unoccupied by the succeed- 

 ing vertebra, forming, apparently, a weak joint. This feature is shown 

 in PI. IX, figs. 2, 3, and 4. 



The discovery of this new form of vertebra shows that the terms 

 opisthocoelian and proccelian, in general use to describe the centra of 

 vertebrae, are inadequate, since they relate to one end only, the other 

 being supposed to correspond in form. The terms convexo-concave, 

 concavo convex, plano-concave, etc., would be more accurate and equally 

 euphonious. 



In Ceratosaurus, as in all the Theropoda except Ocelurus, the cervical 

 ribs are articulated to the centra, not coossifled with them, as in the 

 Sauropoda. The latter order stands almost alone among dinosaurs in 

 this respect, as all the Predentata — Stegosauria, Ceratopsia, and the 

 Oruithopoda — have free ribs in the cervical region. 



The dorsal and lumbar vertebra? are biconcave, with only moderate 

 concavities. The sides and lower surface of the centra are deeply exca- 

 vated, except at the ends, as shown in PI. IX, fig. 5. These vertebrae 

 show the diplosphenal articulation seen in Megalosaurus, and also in 

 Creosaurus, as shown in PI. XII, fig. 5. 



All the presacral vertebrae are very hollow, and this is also true of 

 the anterior caudals. 



There are five well-coossified vertebra? in the sacrum in the present 

 specimen of Ceratosaurus nasicornis. The transverse processes are 

 very short, each supported by two vertebrae, and they do not meet at 

 their distal ends. 



The caudal vertebra' are biconcave. All the anterior caudals, except 

 the first, supported very long chevrons, indicating a high, thin tail, 

 well adapted to swimming (PI. IX, fig. 6). The tail was quite long, 

 and the distal caudals were very short. 



THE SCAPULAR ARCH. 



Tbe scapular arch of Ceratosaurus is of moderate size, but the fore 

 limbs were very small. The humerus is short, with a strong radial crest. 

 The radius and ulna are also very short, and nearly equal in size. The 

 carpal bones were only imperfectly ossified. There were four digits in 

 the fore foot, and all were armed with sharp claws. The second and 

 third digits were much larger than the first and fourth, and the fifth 

 was entirely wanting. 



THE PELVIC ARCH. 



The pelvic bones in the Theropoda have been more generally mis- 

 understood than any other portion of the skeleton in dinosaurs. The 

 ilia, long considered as coi acoids, have been usually reversed in posi- 

 tion; the ischia have been regarded as pubes; while the pubes them- 

 selves have not been considered as part of the pelvic arch. 



